Interferometric microscopes are used extensively in the metrology and quantitative characterization of surfaces. Such devices are non-contact instruments of extraordinary precision having typical vertical solutions of about or in excess of 1 .ANG.. In an interference microscope, an interferometer is typically mounted at the downstream end of or following the microscope objective. There, light--most commonly collimated light from a laser or the like--from the source is split by a beamsplitter into a reference beam and a measurement beam. The two beams travel different paths to the reference and measurement (or test) surfaces, respectively, from which they are reflected back and then recombined at the beamsplitter. The resulting interference pattern is used to extract the relative differences in path length of the two beam paths using well known interferometric techniques. If the reference surface features are known, these path differences can be correlated to the height of surface features on the measurement or test surface, thus providing the user with a three-dimensional perspective or mapping of the measurement surface. See, for example, J. F. Biegen et al., "High-Resolution Phase-Measuring Laser Interferometric Microscope for Engineering Surface Metrology", 1 Surface Topography 469 (1988). However, accurate data can only be obtainable when the measurement surface is in focus.
Accurate focusing of an interferometric microscope using conventional microscope focusing techniques is, for a number of reasons, difficult at best. For example, some prior art methods utilize "texture algorithms", such as are disclosed in U.S. Pat. Nos. 4,600,832 (Grund), 4,577,095 (Watanabe), 4,447,717 (Nohda) and 4,333,007 (Langlais, et al.), to focus on discernible features of the measurement surface. Unfortunately many highly polished surfaces of the type generally measured in precision surface metrology are devoid of features large enough to accommodate the effective use of these techniques.
Another prior art focusing technique, disclosed in U.S. Pat. No. 4,931,603 (Cohen, et al.), relies on the known fact that interferometric microscopes using an equal path (MIRAU) objective exhibit maximum interference at focus. This technique accordingly searches for the extremely narrow interference region from a broadband white light source. However, the extreme narrowness of the interferences region, typically a few microns, makes systematic searching an unusually time-consuming task, even when relatively close to focus. Moreover, once the interference region is located it must still be mapped to determine the point of maximum interference contrast. Thus, this prior art technique has no true "capture range" or region in which the direction of test surface movement toward focus is known, leaving systematic searching for focus as the only viable alternative, Such searching is time consuming and greatly increases the likelihood of a collision through actual physical contact between the objective and target, raising the very real possibility that both may be damaged or destroyed.
Other prior art focusing techniques are based on optical triangulation principles and are commonly referred to as triangulation methods. These include, among others, the skew beam method and the Foucault knife edge method. See, for example, E. H. Hellen and D. Axelrod, "An Automatic Focus/Hold System for Optical Microscopes", 61 Rev. Sci. Instrum. 3722 (1990). The use of similar techniques in conventional microscopes is also disclosed in U.S. Pat. Nos. 4,687,913 (Chaben), 3,798,449 (Reinheimer, et al.), 4,661,692 (Kawasaki) and 4,625,103 (Kitamura, et al.). In these methods, a peripheral ray is reflected from the test surface and is then focused at an image plane. As the test surface moves, the longitudinal focus shift of the image is translated into a lateral motion of the ray across the face of a differential detector located at the image plane, and a focus error signal is generated by the autofocus detector. These methods enjoy high precision, a large capture range, high-speed responsiveness suitable for fast servo control, and simple architecture. Unfortunately, when used in conjunction with an interferometric microscope, the presence of interference produces modulation in the focus error signal and creates multiple zero crossings which are indistinguishable from the single zero crossing that identifies true focus, thus preventing a unique determination of focus. All triangulation methods suffer from this effect when used in interferometric optical systems.
Still another prior art focusing technique is the astigmatic lens method. See, for example, D. Cohen et al., "Automatic Focus Control: the Astigmatic Lens Approach", 23 Applied Optics 565 (1984). In this method, a point source is reflected off the object plane and is refocused at the image place using an astigmatic lens. If a quadrant detector, whose lines dividing the sensing quadrants lie at 45 degrees to the orientation of the astigmatic focal lines, is located at the image plane, a focus error signal similar to that produced by triangulation methods can be obtained. Unfortunately, the astigmatic lens method also suffers from interference effects in precisely the same manner as do triangulation methods.
There is accordingly an unsatisfied need, particularly for use in or in association with interferometric microscopes and the like, for an automatic focusing apparatus and technique that is fast and repeatable with a broad capture range, and that can be utilized for quantitative characterization of the types of surfaces typically encountered or of interest in precision surface metrology.